Cartesian doubt is a form of methodological skepticism associated with the writings and methodology of René Descartes. Cartesian doubt is also known as Cartesian skepticism, methodic doubt, methodological skepticism, or hyperbolic doubt.
Cartesian doubt is a systematic process of being skeptical about (or doubting) the truth of one's beliefs, which has become a characteristic method in philosophy. This method of doubt was largely popularized in Western philosophy by René Descartes (1596-1650), who sought to doubt the truth of all his beliefs in order to determine which beliefs he could be certain were true.
Methodological skepticism is distinguished from philosophical skepticism in that methodological skepticism is an approach that subjects all knowledge claims to scrutiny with the goal of sorting out true from false claims, whereas philosophical skepticism is an approach that questions the possibility of knowledge.
Cartesian doubt is methodological. Its purpose is to use doubt as a route to certain knowledge by finding those things which could not be doubted. The fallibility of sense data in particular is a subject of Cartesian doubt.[dead link]
There are several interpretations as to the objective of Descartes' skepticism. Prominent among these is a foundationalist account which claims that Descartes' skepticism is aimed at eliminating all belief which it is possible to doubt, thus leaving Descartes with only basic beliefs (also known as foundational beliefs). From these indubitable basic beliefs, Descartes then attempts to derive further knowledge. It's an archetypal and significant example that epitomizes the Continental Rational schools of philosophy.
Descartes' method (broken into four "scientific" steps including a. accepting only information you know to be true b. breaking down these truths into smaller units c. solving the simple problems first d. making complete lists of further problems) is also known as hyperbolic doubt or having the tendency to doubt, since it is an extreme or exaggerated form of doubt. (Knowledge in the Cartesian sense means to know something beyond not merely all reasonable, but all possible, doubt.) In his Meditations on First Philosophy (1641), Descartes resolved to systematically doubt that any of his beliefs were true, in order to build, from the ground up, a belief system consisting of only certainly true beliefs. Consider Descartes' opening lines of the Meditations:
Several years have now elapsed since I first became aware that I had accepted, even from my youth, many false opinions for true, and that consequently what I afterward based on such principles was highly doubtful; and from that time I was convinced of the necessity of undertaking once in my life to rid myself of all the opinions I had adopted, and of commencing anew the work of building from the foundation...
— René Descartes , Meditation I, 1641
Descartes' method 
René Descartes, the originator of Cartesian doubt, put all beliefs, ideas, thoughts, and matter in doubt. He showed that his grounds, or reasoning, for any knowledge could just as well be false. Sensory experience, the primary mode of knowledge, is often erroneous and therefore must be doubted. For instance, what one is seeing may very well be a hallucination. There is nothing that proves it cannot be. In short, if there is any way a belief can be disproved, then its grounds are insufficient. From this, Descartes proposed two arguments, the dream and the demon.
The dream 
Descartes, knowing that the context of our dreams, while possibly unbelievable, are often lifelike, hypothesized that humans can only believe that they are awake. There are no sufficient grounds by which to distinguish a dream experience from a waking experience. For instance, Subject A sits at her computer, typing this article. Just as much evidence exists to indicate that her composing this article is reality as there is to demonstrate the opposite. Descartes conceded that we live in a world that can create such ideas as dreams. However, by the end of The Meditations, he concludes that we can distinguish dream from reality at least in retrospect.
The Evil Genius 
Descartes reasoned that our very own experience may very well be controlled by an evil genius of sorts. This genius is as clever and deceitful as he is powerful. He could have created a superficial world that we may think we live in.
In Meditation I, Descartes stated that if one were mad, even briefly, the insanity might have driven man into believing that what we thought was true could be merely our minds deceiving us. He also stated that there could be 'some malicious, powerful, cunning demon' that had deceived us, preventing us from judging correctly.
Descartes argued that all his senses were lying and since your senses can easily fool you, his idea of an infinitely powerful being must be true as that idea could have only been put there by an infinitely powerful being which would have no reason to be deceitful to him.
I think, therefore I am 
While methodic doubt has a nature, one need not hold that knowledge is impossible in order to apply the method of doubt. Indeed, Descartes' attempt to apply the method of doubt to the existence of himself spawned the proof of his famous saying, "Cogito ergo sum" (I think, therefore I am). That is, Descartes tried to doubt his own existence, but found that even his doubting showed that he existed, since he could not doubt if he did not exist.
Cartesian skepticism advocates the doubting of all things which cannot be justified through logic. Some have claimed that the corresponding philosophical proposition fails the criterion of falsifiability that is required of any empirical theory (see, for instance, Karl Popper).
See also 
References and further reading 
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (January 2011)|
- Burnham, D. & Fieser, J. (2006). "René Descartes". The Internet Encyclopedia of Philosophy, J. Fieser & B. Dowden (eds.). <http://www.iep.utm.edu/d/descarte.htm>.
- Cottingham, Stoothoff, and Murdoch (eds.). (1984). The Philosophical Writings of Descartes. Cambridge: Cambridge University Press.
- Descartes, René. (1641). Meditations on First Philosophy. In Cottingham, et al. (eds.), 1984. Online at <http://www.wright.edu/cola/descartes/>.
- Newman, Lex. (2005). "Descartes' Epistemology". The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.). <http://plato.stanford.edu/entries/descartes-epistemology/>.